A light-emitting diode (LED) is a solid state semiconductor device that includes one or more p-n junctions. LEDs emit light when current flows through the p-n junctions thereof. Blue light emitting LEDs are in wide use today and are typically formed by growing Group III-nitride semiconductor layers (e.g., gallium nitride based layers) on a silicon carbide, sapphire or gallium nitride substrate. The brightness and energy efficiency of the light emitted by an LED may be directly related to the amount of an operating or “drive” current that flows through the p-n junction of the LED. Typically, an LED is designed to operate at a drive current level that provides both high brightness and high energy efficiency.
Most LEDs are nearly monochromatic light sources that appear to emit light having a single color. Thus, the spectral power distribution of the light emitted by most LEDs is tightly centered about a “peak” wavelength, which is the single wavelength where the spectral power distribution of the LED reaches its maximum as detected by a photo-detector. The “width” of the spectral power distribution of most LEDs is between about 10 nm and 30 nm, where the width is measured at half the maximum illumination on each side of the peak of the spectral power distribution (this width is referred to as the “full-width-half-maximum” width).
In order to use LEDs to generate white light, LED-based light emitting devices have been provided that include several LEDs that each emit a light of a different color. The different colored light emitted by the LEDs combine to produce white light. For example, by simultaneously energizing red, green and blue LEDs, the resulting combined light may appear white, or nearly white, depending on, for example, the relative intensities, peak wavelengths and spectral power distributions of the red, green and blue LEDs.
White light may also be produced by coating, surrounding or otherwise associating an LED (e.g., a blue or ultraviolet light emitting LED) with one or more phosphors that convert some of the light emitted by the LED to light of one or more other colors. For example, a white light emitting LED package may be formed by coating a gallium nitride-based blue LED (i.e., an LED that emits blue light) with a “yellow” phosphor (i.e., a phosphor that emits light having a peak wavelength in the yellow color range) such as a cerium-doped yttrium aluminum garnet phosphor, which has the chemical formula Y3Al5O12:Ce (YAG:Ce). The combination of the light emitted by the blue LED that is not converted by the phosphor and the green, yellow and orange light that is emitted by the broad-spectrum YAG:Ce phosphor may be perceived by a human observer as white or near-white light. The term “phosphor” is used broadly herein to refer to a material that absorbs light in a first wavelength range and in response thereto emits light in another wavelength range (typically longer wavelengths). Typically, particles of a phosphor are mixed into a binder material such as, for example, an epoxy-based or silicone-based curable resin, and this mixture is then coated, sprayed or poured onto an LED and/or another surface of a light fixture. Herein, such phosphor-including mixtures are referred to as a “recipient luminophoric medium.”
Initially, LEDs were primarily used in specialty lighting applications such as providing back-lighting and/or indicator lights in electronic devices. As the light output and energy efficiency of LEDs has improved, LEDs have been used to form solid state light fixtures such as LED-based light bulbs, downlights, ceiling mounted “troffer” light fixtures that are used as replacement for conventional fluorescent light fixtures, streetlights and the like. As used herein, the term “solid state light fixture” refers to a packaged lamp, light bulb or other light fixture that includes a plurality of LEDs.
Solid state light fixtures generate less heat, are far more energy efficient and have far longer lifetime as compared to incandescent light bulbs. Solid state light fixtures also exhibit numerous advantages over fluorescent light bulbs, including better energy efficiency, faster turn-on and longer lifetimes. Solid state light fixtures may also generate more aesthetically pleasing light than fluorescent light bulbs, and do not contain mercury. Because of these advantages, solid state light fixtures are increasingly replacing conventional incandescent and fluorescent light bulbs in numerous applications including general illumination applications such as lighting for homes and offices. As solid state light fixtures are used in a much wider array of applications, the ability to efficiently and effectively dim solid state light fixtures (i.e., reduce the overall output or “brightness” of the emitted light) has arisen as an issue as consumers expect many different types of light fixtures to have dimming capabilities.